On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff.
So the warning message could have potentially prevented both crashes and anything like it (assuming it activated during the takeoff roll, before V1 and the pilots were trained to abort take off when it happened)
Though you are right, if the AoA sensor had failed halfway though a flight, they still would have had problems.
> On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff.
Are you sure? The AvHerald narrative for the Ethiopian Airlines flight:
"The takeoff run was entirely normal, both AoA sensors were in agreement. Shortly after becoming airborne the left AoA sensor however began to deviate and reached a position of about 85 degrees nose up, which obviously triggered the left hand stick shaker motor (and provides a very noisy and distracting cockpit environment), which in turn, different to the 737 NG behaviour (where the AoA does NOT correct the pitot data), also caused the IAS and ALT data to differ from the right hand system, in particular the left IAS became 12 knots higher than the right hand IAS (the preliminary report does not mention that the IAS DISAGREE warning activated however, which would invoke the unreliable airspeed procedures). The captain was pilot flying and focussed to keep the aircraft on track (via the flight director) due to terrain around while at the same time trying to understand why the stick shaker activated. Several required callouts by the first officer did not occur (e.g. speed, ...), as result the crew lost speed as well as thrust control completely out of sight."
As I said somewhere below, I don't think the AoA disagree indication would have made much of a difference here.
The new information in the article (buried in the last part of it!) is that apparently the warning message just didn't work on the ground at all, even with the already non-functioning sensors, and even if that option was paid for; read it again:
"We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground" ...
"But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground."
Knowing nothing about these things, are the AoA sensors accurate on the ground? I assume they depend on predictable airflow, of which there's initially none and Boeing have to draw the line somewhere as to when they become reliable?
Even if they're accurate, due to the ground effect[0] when an airplane is close to the ground it can have significantly higher AoAs without stalling because the stalling speed is so low at those heights.
> when an airplane is close to the ground it can have significantly higher AoAs without stalling
No. A wing will stall at a consistent critical angle of attack. Ground effect works by increasing the lift available for a given angle of attack, not by allowing a larger angle of attack to develop.
Parent comment was asking about the need for minimum airspeed to set the AoA sensors into a reliable state such that they could be checked at the gate, though, which is unrelated to ground effect.
If the issue you're worried about is only MCAS related, it seems like the takeoff roll might be a poor place to want to detect it.
An abort just prior to V1 is a no-joke exercise from a safety standpoint. MCAS operation is suppressed with any flaps deployed, so taking an airplane with a failed AoA sensor into the air and troubleshooting/returning to the departure airport if needed or simply flying with an INOP MCAS may very well be safer than a high-speed abort.
Good point. Instead of aborting take-off, either do a circuit back WITHOUT retracting flaps, or set Stab Trim switch to CUTOUT and fly to destination trimming manually with the trim wheel? Wonder what pilots are briefed to do currently (well, if MAXes were flying).
So the warning message could have potentially prevented both crashes and anything like it (assuming it activated during the takeoff roll, before V1 and the pilots were trained to abort take off when it happened)
Though you are right, if the AoA sensor had failed halfway though a flight, they still would have had problems.